auditory and vestibular system - eötvös loránd …detari.web.elte.hu/english...
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Auditory and vestibular system
Sensory organs on the inner ear• inner ear: audition (exteroceptor) and vestibular
apparatus (proprioceptor)• bony and membranous labyrinths within the
temporal bone (os temporale) ����
• both sensory organs contain hair cells: secondary receptor cells
• these sensory systems are related evolutionary with the lateral-line organ (fish, amphibians) –acousticolateral organ
• structure of hair cells is similar everywhere• hair cells are flanked by supporting cells – tight
junctions: perilymph and endolymph separated• perilymph: extracellular space, high Na+, low K+
• endolymph: transcellular fluid, high K+, low Na+
• endolymph is positive compared to perilymph –150 mV compared to the intracellular space
• at the tip of the stereocilia mechanosensitive K+
channels, 10-15% open - 90 Hz firing rate on primary axons – changes depend on direction of bending 2/14
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hair cell
stereocilium
Structure of hair cells
supportingcell
supportingcell
endolymphhigh K+
perilymphhigh Na+
+ 80 mV
+ 0 mV
- 70 mVK+Na+
K+ Na+
ATP
K+
actin
Ca+
basal connection
tip-linkkinocilium
transmitter
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Cilia on hair cells
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2002, Fig. 7-24
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Vestibular system I.• vestibular apparatus consists of two parts:
– 3 semicircular canals – detect angular acceleration– otolith organs (utriculus, sacculus) – detect position
of the head, linear acceleration
• semicircular canals are approximately perpendicular to each other
• horizontal one is tilted downwards by 25°, the two vertical ones have an angle of 41° and 56°, respectively with the sagittal plane
• sensory epithelium with the hair cells is called crista ampullaris, it is located at one end of the canal in the ampulla
• hair cells are covered by the cupula that almost blocks the flow of the endolymph ����
• lateral-line organ works similarly ����• kinocilia are uniformly oriented in the crista• semicircular canals are in pairs within the
same plane - complementary pairs – when one is excited, the other is inhibited
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Vestibular system II.
• hair cells are located in maculas within the utriculus and sacculus
• macula is horizontal in the utriculus, while it is vertical in the sacculus
• hair cells are covered by otolith membranecontaining otoliths (calcium carbonate) ����
• kinocilia in maculas (maculae) are oriented with respect to an undulating line, the striola
• any position of the head causes a specific firing pattern in the primary afferents
• linear acceleration (lift) is also detected because of the inertia of the otoliths
• primary sensory neurons are located in the ganglia vestibulare
• central axons run to the four vestibular nuclei in the brainstem
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Vestibular centers
• Deiters’ nucleus (nucl. vestibularis lateralis) – strong, tonic excitatory effect on spinal motor
neurons – tractus vestibulospinalis lateralis
– crucial for the maintenance of straight posture
– opposite effects: cerebellar inhibition on Deiters’ nucleus, cortical (in tetrapods rubral) inhibition on spinal motor neurons
– decerebration rigidity
• nucl. vestibularis medialis– brief, short lasting input from semicircular canals –
control of neck muscles
• nucl. vestibularis superior– similar input – control of eye movements
• nucl. vestibularis inferior– less known, it integrates inputs with cerebellar
information and provides ascending pathways
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Audition• ear is one of the most important telereceptive
organ – vision is limited by darkness, olfaction depends on the direction of the wind
• audition is functioning even during sleep – weak sounds made by the baby awake the mother
• audition is very important for communication too• sounds are longitudinal waves• human ear is sensitive to sounds between 20 Hz
and 20 kHz, some animals hear ultrasounds (rat after coitus); infrasound is annoying
• intensity in given as the logarithm of the ratio to a reference value (20 µPa – threshold at 2 kHz) because the range of intensities is wide
• in practice tenth of Bel, decibel (dB) is used, thus the logarithm should be multiplied by 10
• if voltage or current is used, then the second power of these values should be entered in the equation – that’s why we use 20 and not 10 in the equation
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The organ of audition• human ear consists of 3 parts: external, middle
and inner ear
• external ear:– pinna (can be moved in some animals)
– auditory canal
– tympanic membrane – closing external ear
• middle ear:– ossicles (malleus, incus, stapes) – 22-fold increase in
pressure because the different surface size and the lever system
– Eustachian tube to the pharynx – equalization of pressure in the external and middle ears (flying, yawning, candy)
• inner ear:– bony and membranous labyrinth with three canals
– scala media (ductus cochlearis) surrounded by membrana basilaris (with the organ of Corti) and Reissner’s membrane
– above: scala vestibuli, below: scala tympani ����
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Mechanism of audition I.• ossicles forward sound stimuli to the perilymph
(scala vestibuli) through fenestra ovale• cochlea has 2,5 turns – sound waves reach the
top of the cochlea, then return through scala tympani to fenestra rotunda
• total length of the cochlea is about 32-33 mm• sound waves also reach inner ear by bone
conduction – it is less important, except when listening to our own sound record, or when middle ear is damaged and hearing aid is needed
• the site of maximal oscillation of the membrana basilaris depends on frequency – tonotopy
• movement of membrana basilaris stimulates hair cells ����
• membrana basilaris is narrow and tight at the base (100µ), and wide and loose at the top (500µ)
• Helmholtz suggested, György Békésy proved that high pitch tones are detected at the base, low pitch tones at the top of the cochlea
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Mechanism of audition II.
• function of inner and outer hair cells is different• inner hair cells – sensation, outer hair cells -
setting sensitivity• activation induces shortening of outer hair cells
due to activation of their cytoskeleton –amplitude of maximal oscillation increases
• threshold of inner hair cells is higher, it reaches normal detection limit only due to amplification
• noise-induced hearing loss: (Walkman), and certain medicines (streptomycin) destroy outer hair cells
• sterocilia are also shorter and tighter at the base of the cochlea – contributes to tonotopy
• outer hair cells reach tectorial membrane, inner ones not – turbulently flowing perilymphstimulates them
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Mechanism of audition III.• mechanosensitive channels of stereocilia respond
with time resolution in the µs range – K+ enters, because of the 150 mV potential difference
• depolarization causes Ca++ influx – shape change in outer hair cells, increased glutamate (?) release in inner hair cells
• primary sensory neurons are located in the ganglion spirale – 1 hair cell 10 afferents, 1 afferent – 1 hair cell
• there are approx. 3500-3500 hair cells, i.e. 60-70000 afferents on the two sides together
• afferents cannot follow 20 kHz frequency, spikes appear phase-locked
• intensity is coded partly in frequency, partly in the number of cells activated (population code) depending on recruitment of neighboring and high-threshold hair cells
• sensitivity is set by the lateral (inner hair cells and primary dendrites) and medial (outer hair cells) olivocochlear bundle
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Central auditory pathway I.• onset and termination of sounds, location of
the source, and stimulus pattern should be analyzed by the central apparatus
• anatomy is known, physiology is less well • general characteristics: parallel ascending
fibers, two-directional connections, tonotopy• first relay stations are the cochlear nuclei
(anteroventral, posteroventral and dorsal) –strictly ipsilateral
• deafness restricted to one side is either because of damage to this nuclei or disruption at the periphery
• primary afferents related to one hair cell (approx. 10 axons) terminate in one layer
• fibers originating in these nuclei, join ipsi- and contralateral lemniscus lateralis and go to inferior colliculus, or reach superior olivary nucleus – second relay station
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Central auditory pathway II.
• oliva receives bilateral projection, it localizes sound source – based on differences in phase [below 2 kHz], or in intensity [above 2 kHz]
• 1° difference in direction can be detected
• fibers from oliva join lemniscus lateralis and run to the third relay station, inferior colliculus
• inferior colliculus is also important in detection of sound direction – e.g. owls
• output to non-auditory areas as well
• fourth station is corpus geniculatum mediale(medial geniculate body) – tonotopic representation, input from other sensory areas as well
• the end station is the primary auditory cortex,Br. 41-42 in the temporal lobe – several areasarranged in a tonotopic way
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Structure of the inner ear
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2002, Fig. 7-27 a,b.
Crista ampullaris and macula
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2002, Fig. 7-27 c,d.
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The lateral-line system
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2002, Fig. 7-25
Structure of the labirynth
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2002, Fig. 7-29.
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Organ of Corti
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2002, Fig. 7-30 a,c.